The huge bandwidth available in optical fiber in the low attenuation band is not accessible through electronic interfaces unless some type of multiplexing is used. Wavelength Division Multiplexing (WDM) offers the most efficient method to exploit the available bandwidth. In this technology, a number of parallel wavelength channels are used, where each channel carries up to a maximum data rate accessible through electronic interfaces. Moreover, the data protocols, framings and data rates used on different channels are totally independent of each other. As the technology progresses the number of feasible channels in the total band is increasing. The early WDM systems only used 4 to 16 channels while new systems are targeting higher number of channels, and are hence called Dense WDM (DWDM).
The low attenuation wavelength region is partitioned into smaller wavelength bands. The first band used in modern optical communications is called the Conventional Band or C-Band. This band included wavelength channels from 1520 to 1565 nm. Wavelengths covering 1565 to 1610 nm form the Long Band or L-Band, while 1475 to 1520 nm is called the Short Band or S-Band.
At the transmitter side of a DWDM system, a large number of different laser sources with different wavelengths are required. Each data stream is modulated on one of the wavelength channels and all the wavelength channels are multiplexed and sent to the same optical fiber. At the receiving end, each channel is demultiplexed from the set of wavelength channels. An optical receiver, then, will demodulate data from each channel. The capacity of a DWDM system increases as more wavelength channels are established. As a result, it would be desirable to increase the number of channels, decrease the channel spacing and increase the total wavelength window.
The DWDM systems now need a large number of laser sources as well as techniques to modulate a data signal on each source, combine, demultiplex and detect each data stream.
Currently, the laser sources designed into DWDM systems are exclusively of the single-wavelength variety. Distributed Feed-Back (DFB) lasers, Fabry-Perot lasers and ring lasers are some of the main technologies. Each wavelength supported in the system has a dedicated laser and its ancillary electronics. In the last few years and still today, the majority of lasers used are capable of emitting light only at a fixed wavelength. Increasingly, however, designs are making use of tunable wavelength lasers, which have broader spectral range and can operate at any point within that range. The key drawback of these devices however, is the sheer number that are required to satisfy the high channel count systems being proposed for the future optical network. At the same time, it is very important to be able to lock the center wavelength of each laser source to a specific wavelength. This is mainly because of the fact that if there is any drift in the wavelength of a laser, it can interfere with the adjacent wavelength channel communications. This imposes a practical limitation on the number of discrete laser sources that can be placed in a very tightly spaced wavelength channel system to realize a large number of channels. As a result, a multi-wavelength laser source that can provide an efficient and simple wavelength locking system is highly needed.
Although felt particularly acutely in the areas of system source architecture, the pressure to adapt to a high channel count reality is felt in other related areas. As increasingly tight channel spacings are supported for example, a new generation of instrumentation equipment is required to address the need for the characterization of performance and behavior with such fiber loads. This need extends the full length of the supply chain as component suppliers are required to quantify operation of advanced products during development and manufacturing, as system vendors develop, optimize, and validate equipment response to real-world scenarios, and as service providers qualify equipment and test out vendor claims.